Rotor windings for turbo-generators are conventionally produced as a rule from soft copper, in order to permit bending of the conductors. In the case of relatively small generators in particular, groups of soft copper conductor bars are used for the rotor winding, bent subsequently bundle by bundle into the desired shape.
In the case of large generators, however, there is the problem that the high centrifugal forces cause rotor windings of soft copper to be displaced and deformed and therefore this is no longer a suitable material. Therefore, soft copper conductor bars have been replaced by hard-drawn copper conductor bars. With these hard-drawn copper conductor bars, however, it had then been necessary to solve the problem concerning how the required curvature of these conductor bars for producing rotor windings can be achieved.
Therefore, it was initially attempted to apply to hard-drawn copper conductor bars a bending method analogous to that used for bundles of soft copper conductor bars. However, problems were encountered when bending bundles of hard-drawn copper conductor bars. Owing to the fact that the individual hard-drawn copper conductor bars spring back differently after bending--by contrast with the conventional soft copper conductor bars--it was not possible to bend a compact bundle into a desired shape. Due to this different springing back of the individual hard-drawn copper conductor bars, the need arose for re-working, or re-bending, the individual hard-drawn copper conductor bars until a bundle of the desired shape is finally obtained. However, such a method would be far too laborious for the production of rotor windings and is consequently too cost-intensive. Furthermore, owing to the different behavior of the individual hard-drawn copper conductor bars, it is difficult to maintain reliably a required degree of accuracy.
Therefore, in the state of the art a compromise is currently accepted to improve stability. To be specific, in order to obtain curvatures, individual rotor windings are joined together from hard-drawn copper conductor bar segments, in order to achieve greater stability of the rotor windings with respect to centrifugal forces. These hard-drawn copper conductor bar segments are in this case connected by means of a corner soldered joint, at which the greatest axial stressing due to thermal expansion takes place, in order to achieve the curvature required for the rotor winding.
However, with this corner soldered joint, the heating effect during the soldering operation makes the hard-drawn copper turn into soft copper in the region of the soldering location. This has the result of producing in the regions of the soldering locations soft regions in which the rotor windings therefore have less stability with respect to centrifugal forces, even if this structure is more stable in comparison with conventional soft copper conductor bars. What is more, corner soldered joints are laborious to produce.